Although simple in principle injection procedures based on syringe type devices with injection needles require mastering of several discrete steps. Before the mere injection procedure some initiation actions may be required. Filling the syringe with medication withdrawn from a reservoir such as a vial may be needed, taking into account the proper dose to be administered. In order to avoid this step in the actual treatment situation it is common to provide pre-filled syringes, in which case, however, a dose setting or selecting step may be needed. In its first movement the syringe piston may need an extraordinary break-loose force after storage to overcome both internal reshaping resistance and an increased wall friction due to adherence or depletion of lubricant in contact points. For storage and shelf life reasons pre-filled syringes sometimes are delivered in dual or multiple-chamber form, requiring an additional mixing step immediately before treatment. De-aeration and pre-ejection are generally needed to remove gas in the vessel compartment and fill out spaces for example at the front sealing, outlet attachments and the interior of the outlet devices or needles. Syringes containing multiple doses further requires repeated ejection of predetermined volumes under avoidance of both over and under dosing, overdosing generally being irreversible and under-dosing often being undetected or unavoidable when ejecting an insufficient last dose from the syringe. Proper sequencing of all these steps are crucial to a safe and non-traumatic treatment.
These demands can be met also when using the simplest injection devices, such as the common hypodermic syringe, when in the hands of a skilled operator who also may initiate medically relevant corrective measures in case of accidents and malfunction. However, a general treatment trend is to place administration responsibility on the patients themselves, also in the case of child, elderly and disabled persons. In long-term treatment the patient often develop a certain skill but less frequent administration schemes also exist, often including situations of emergency or patient imbalance. Other unique problems in patient selfadministration, as compared to assistant operated administration, is that less suitable and often strained body positions are required and that apprehended or experienced pain or discomfort may interfere with the medically desirable action pattern. In summary, especially the selfadministration requires more sophisticated devices to facilitate the injection procedure and avoid or reduce risks for mistakes. Patients dependent on daily or occasional administrations also have a legitimate need for convenience and devices discrete enough to be brought around in daily life. Yet it is desirable that such sophistication and convenience is kept simple and inexpensive to allow for widespread distribution and inclusion also in disposable devices.
Various proposals have been put forward for injector devices assisting the user in many of the steps outlined. While meritorious in some respects none seem to deal with more than part of the total sequence. The U.S. Pat. No. 5,244,465 patent specification describes limiting arrangements for securing a single ejection of one of several selectable dose volumes. The U.S. Pat. No. 4,050,459, GB 1230522 and DE G8509572 specifications describe various trail for securing repeated multiple equal doses of predetermined volume. The U.S. Pat. Nos. 3,517,668 and 5,807,346 patent specifications and the PCT/CH96/00115 application describe arrangements for multiple ejections of selectable dose volumes. The U.S. Pat. No. 4,832,694 specification describes a trail arrangement for a single aspiration and ejection cycle under prevention of any reverse movement. The specifications referred to do not give any solutions to the earlier phases in injection sequence or how to integrate such phases. The U.S. Pat. Nos. specifications 4,968,299, 4,874,381, 5,080,649 and 5,728,075 and WO 93/14799 all relates to arrangements for securing mixing of contents in two chamber syringes followed by injection, utilizing a screw-threaded mechanism for the mixing phase. Again these suggestions only relate to a part of the overall operation.
Accordingly there is a continuing need for simple and inexpensive injection devices able to assist the user in the various handling steps involved, preventing or ameliorating mistakes and offering an ergonomic, convenient and non-traumatic product, especially useful for patients under self-administration. Although the present invention may have a more general utility, it will mainly be described against this background.